Dual-tone multifrequency (DTMF) signaling conveys information over the telephone network. DTMF tones—commercially known as “touch-tones”—are often used to communicate a telephone number to a switch, but they are also becoming an important component in the growing use of Interactive Voice Response (IVR) systems.
IVR systems are a common method for an initial interface with a user at a call center. Before (and if at all) a user is routed to an agent, the user may be prompted to enter some details that identify the user and the user's specific query. The entry is typically performed using the telephone keypad.
There are sixteen different DTMF tones (defining digits 0-9, #, *, and A, B, C, D) defined using combinations of eight different single-frequency tones. As telecommunication devices and protocols progress and develop, the definition for generating DTMF tones also changes depending on the device type. So-called “plain old telephone service” (POTS) landline phones, Global System for Mobile Communications (GSM) cell phones, and Voice over Internet Protocol (VOIP) phones all handle DTMF differently. Furthermore, as DTMF tones traverse the public switched telephone network (PSTN) network, they will be modified slightly due to noise and other effects due to the communication channel. In most cases, these modifications are gentle enough not to affect the DTMF detection algorithms that map the audio signal to its corresponding value (e.g. 0-9, #, *, and A, B, C, D).
As recognized by the inventors, the audio signal of DTMF tones generated by a far end user will exhibit deviations from an “ideal” DTMF tone when observed at the near end of a call. This discrepancy will depend on the type of device used and on the relative geographical locations of the near and far end users. The ideal DTMF tone is known a priori, and by calculating various statistical entities based on the difference between the ideal DTMF tone and the observed, the device type and the relative geographic location of a user can be provided.
As recognized by the inventors, different makes and models of telephones, smartphones, and softphones often have uniquely identifiable DTMF tones. By monitoring the tones of a known device type, phone number, or user, future tones can be used to determine the device type or authenticity of the phone number or user.